In the fourth generation mobile communication system, HARQ attracts attention as a data retransmission method. In HARQ, in order to develop tolerance in a radio propagation path, a packet composed of transmitted data and error correcting bits (parity bits) generated by turbo encoding is divided into multiple subpackets for radio communication and are transmitted and received on a subpacket-by-subpacket basis. Usually, data is transmitted in the first subpacket and parity bits are transmitted in the subsequent subpacket. After transmitting a subpacket, a data transmission node waits a response from the data reception node and determines a subpacket to be transmitted next. If a received subpacket is successfully decoded, the data reception node sends back an ACK (Acknowledgement) to the data transmission node; if decoding fails, the data reception node sends back an NAK (Negative Acknowledgement). At this time, the data reception node buffers the received subpacket that was unsuccessfully decoded, and waits to receive the subsequent subpacket.
Upon reception of the NAK from the data reception node for the first subpacket, the data transmission node transmits the second subpacket. When the data reception node receives the second subpacket, it tries to decode the received data by combining the first and the second subpacket. That is, the data reception node tries to decode a data part of the first subpacket already received by using error correcting bits (parity bits) that were received in the second subpacket. The data reception node sends back an ACK or NAK to the data transmission node according to a decoding result.
When the data transmission node received the NAK from the data reception node as a response to the second subpacket, it transmits a third subpacket. In the case where the error correcting bits (parity bits) are divided in multiple subpackets, a remaining part of the error correcting bits is transmitted in the third subpacket. In this case, the data reception node that received the third subpacket tries to decode the received data by combining the first, the second, and the third subpacket.
The data transmission node of an HARQ method is equipped with a repetition function to retransmit an already transmitted packet when receiving a NAK for the last subpacket. Therefore, upon reception of a NAK for the last subpacket, the data transmission node retransmits a subpacket group that has already been transmitted, starting from the first subpacket, sequentially and waits for ACK to be transmitted from the data reception node. In this description, transmission of a subpacket that the data transmission node conducts upon reception of a NAK is called retransmission regardless whether the subpacket is new or has already been transmitted.
Upon reception of an ACK from the data reception node, the data transmission node determines that the packet is successfully transmitted, and transmits a new data packet that should be transmitted next to the data reception node in the above-mentioned subpacket form. When send back of a NAK from the data reception node was repeated and the number of retransmission of the subpacket reaches a previously determined limit value, the data transmission node stops a retransmission operation of subpacket. In this case, it means that packet transmission failed; the higher layer of the data reception node determines a repeat request for the packet, or discard of the received packet, or the like.
The HARQ method in which a transmitted packet including parity bits is configured to be transmitted after being divided into several subpackets enables the received data to be successfully restored before all the parity bits are received, provided that a radio channel state is good, and consequently makes possible a data communication that effectively uses radio resources. In HARQ, a dedicated channel for transmitting ACK/NAK information is prepared in a physical layer, for example, a channel group that includes a certain number of subcarriers of OFDM (Orthogonal Frequency Division Multiplexing), so that the speed of a subpacket retransmission control described above is enhanced.
A radio system using HARQ is proposed, for example, by IEEE802.20 that is a standardization organization, which defines a data retransmission control system by IEEE C802.20-06/04 7.1.3.1.2H-ARQ interlace structure. 3GPP that is a standardization organization is proposing a radio system using HARQ as LTE (Long Term Evolution), and defines a data retransmission control system in the above-mentioned radio system by 3GPP TR 25.814 V7.0.0 (2006-06) 7.1.2.3 HARQ, 9.1.2.5 HARQ. Moreover, 3GPP2 that is a standardization organization proposes a radio system of OFDMA (Orthogonal Frequency Division Multiple Access) using HARQ as LBC (Loosely Backwards Compatible), and defines a data repeat control method in the above-mentioned radio system by 3GPP2 C30-20060731-040R4. 1.3.4 Timeline.